If you receive this kind of error : "certificate verification failed for smtp.sendgrid.net[IP]:587: untrusted issuer /L=ValiCert Validation Network/O=ValiCert, Inc./OU=ValiCert Class 2 Policy Validation Authority/CN=certification validator website"
You can either (a) safely ignore it, or (b) download the Godaddy CA bundle from https://certs.godaddy.com/anonymous/repository.seam
the connection is still encrypted, it's just that your server doesn't have the necessary CA (ceritificate authority) certs to confirm that our certificate is valid.
You specifically will want gd_bundle.crt and something like this:
smtpd_tls_CAfile = /etc/postfix/ssl/gd_bundle.crt in the main.cf.
If the mailserver communicates with more than just us, You'll want to add the above to your existing CA bundle (frequently called ca-bundle.crt).